Dielectric ceramic composition

ABSTRACT

A dielectric ceramic composition consists essentially of 80 to 99.7 wt % of a main component of a system, SrTiO 3  -PbTiO 3  -CaTiO 3  -Bi 2  O 3  -TiO 2  -SnO 2 , at least one oxide of rare earth elements incorporated therein as an additive in an amount of 0.1 to 5.0 wt % in terms of Re 2  O 3  (where Re is at least one rare element selected from the group consisting of Nd, La, Ce, Pr and Sm), and 0.2 to 15 wt % of a vitreous component. The main component consists essentially of 20.0 to 50.0 wt % of SrTiO 3 , 8.0 to 37.6 wt % of PbTiO 3 , 3.2 to 33.9 wt % of CaTiO 3 , 4.4 to 35.2 wt % of Bi 2  O 3 , 2.5 to 13.6 wt % of TiO 2  and 0.2 to 12.0 wt % of SnO 2 . The vitreous component consists essentially of 10 to 45 mol % of Li 2  O, 5 to 40 mol % of at least one oxide selected from the group consisting of BaO, MgO, CaO and SrO, 0.2 to 10 mol % of Al 2  O 3 , 30 to 70 mol % of at least two oxide selected from the group consisting of SiO 2 , MnO 2  and TiO 2 , the content of SiO 2  in the vitreous component being not less than 15 mol % at least, and 1 to 35 mol % of CuO.

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates to a dielectric ceramic composition and, more particularly, a high permittivity dielectric ceramic composition.

2. Description of the prior art

As a high permittivity dielectric ceramic material for capacitors, there have widely been used dielectric ceramic compositions of a system BaTiO₃. Other dielectric materials presently used are compositions of a system, SrTiO₃ -PbTiO₃ -Bi₂ O₃ TiO₂ or SrTiO₃ -PbTiO₃ -Bi₂ O₃ -TiO₂ -CaTiO₃. Such compositions are disclosed in Japanese patent lay-open No. 52-41880 or U.S. Pat. No. 3869398.

However, the composition of the BaTiO₃ system gives a high distortion factor when applied to ceramic capacitors. For example, the composition gives values for third harmonic distortion of -50 to -75 dB when it is applied with a voltage of 10 to 50 Vrms/mm.

On the other hand, the composition of the system SrTiO₃ -PbTiO₃ -Bi₂ O₃ -TiO₂ or SrTiO₃ -PbTiO₃ -Bi₂ O₃ -TiO₂ -CaTiO₃ makes it possible to produce capacitors with a value of third harmonic distortion smaller than that of the BaTiO₃ compositions. However, such a composition has a large dependency of distortion factor on AC voltage, as compared with other capacitors such as film capacitors and aluminum electrolyte capacitors. In addition, when such a composition is applied to multilayer-layer ceramic capacitors, it is required to use noble metals such as platinum as a material for internal electrodes because of high sintering temperature of the composition of 1300° to 1350° C. The use of noble metals results in increase of the manufacturing cost of capacitors.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a dielectric ceramic composition having a low distortion factor and a relatively low sintering temperature of not more than 1200° C., in addition to a high dielectric constant of not less than 500, a small dielectric loss factor of not more than 0.5% at 1 KHz, and a small change rate of capacitance on temperature.

These and other objects of the present invention are achieved by providing a dielectric ceramic composition consisting essentially of 80 to 99.7 wt % of a main component of a system SrTiO₃ -PbTiO₃ -CaTiO₃ -Bi₂ O₃ -TiO₂ -SnO₂, 0.1 to 5.0 wt % of at least one oxide of rare earth elements in terms of Re₂ O₃, where Re is at least one rare earth element selected from the group consisting of Nd, La, Ce, Pr and Sm, and 0.2 to 15 wt % of a vitreous component,

said main component consisting essentially, by weight, of 20.0 to 50.0 % of SrTiO₃, 8.0 to 37.6% of PbTiO₃, 3.2 to 33.9% of CaTiO₃, 4.4 to 35.2% of Bi₂ O₃, 2.5 to 13.6% of TiO₂, and 0.2 to 12.0% of SnO₂,

said vitreous component consisting essentially of 10 to 45 mol % of Li₂ O, 5 to 40 mol % of at least one oxide selected from the group consisting of BaO, MgO, CaO and SrO, 0.2 to 10 mol % of Al₂ O₃, 30 to 70 mol % of at least two oxides selected from the group consisting of SiO₂, MnO₂ and TiO₂, and 1 to 35 mol % of CuO, the content of SiO₂ in the vitreous component being not less than 15 mol % at the least.

The composition of the present invention has been limited to those consisting essentially of 80 to 99.7 wt % of the main component of a system SrTiO₃ -PbTiO₃ -CaTiO₃ -Bi₂ O₃ -TiO₂ -SnO₂, 0.1 to 5.0 wt % of at least one oxide of rare earth elements in terms of Re₂ O₃, where Re is at least one rare earth element selected from the group consisting of Nd, La, Ce, Pr and Sm, and 0.2 to 15 wt % of the vitreous component for the following reasons: If the content of rare element oxide (Re₂ O₃) is less than 0.1 wt %, the dielectric loss factor becomes larger than 0.5%. If the content of rare earth element oxide exceeds 5.0 wt %, the relative dielectric constant becomes less than 500. The incorporation of vitreous component provides lowering of the sintering temperature of the ceramic. However, if its content is less than 0.2 wt %, the sintering temperature becomes higher than 1200 ° C. If the content of vitreous component exceeds 15.0 wt %, the ceramic is made into porous and the dielectric constant becomes less than 500.

The dielectric ceramic composition of the present invention has a high dielectric constant of not less than 500, a dielectric loss factor of not more than 0.5% at 1 KHz, a low distortion factor and a small change rate of capacitance on temperature. In addition, it can be filed at a relatively lower temperature of not more than 1200° C. as compared with the dielectric ceramic composition of the prior art.

The above and other objects, features and advantages of the present invention will become apparent from the following description in connection with several examples.

EXAMPLE 1

There were used SrCO₃, Pb₃ O₄, CaCO₃, Bi₂ O₃, TiO₂, SnO₂, Nd₂ O₃, Sm₂ O₃, La₂ O₃, CeO₂ and Pr₆ O₁₁ as raw materials for a basic composition composed of the main component and at least one oxide of rare earth elements. These raw materials were weighed, mixed in proportions shown in Table 1, and milled by the wet process for 16 hours in a polyethylene pot with alumina balls. After dehydration and drying, the mixture was placed in a zirconia saggar, calcined at 950° C. for 2 hours, crushed and then ground to prepare calcined powder.

Separate from the above, using Li₂ Co₃, BaCO₃, MgO, CaCO₃, SrCO₃, SiO₂, MnO₂, TiO₂, CuO and Al₂ O₃ as raw materials, there were prepared vitreous compositions in the following manner: The raw materials were weighed, mixed each other in proportions shown in Table 2, and milled with a ball mill by the wet process for 16 hours and then dried by evaporation. The resultant mixed powder was put into an alumina crucible, maintained at 1300° C. for 1 hour, vitrified by rapid cooling, and then ground to prepare powder of the vitreous composition having such particle size that particles may pass through a 200 mesh sieve screen.

The above calcined powder was added with the glass composition of Table 2 in proportions shown in Table 1, and then mixed together with a suitable amount of an organic binder by the wet-process in a polyethylene pot for 16 hours. The resultant mixture was dried by evaporation, granulated, and then compacted into disks with a diameter of 10 mm and a thickness of 1.2 mm. The thus prepared green ceramic disks were fired at various temperatures ranging from 940° to 1240° C. for 2 hours to produce ceramic disks.

For each composition, some of the ceramic disks were immersed in a fuchsin solution to determine the optimum firing temperature for each composition. The optimum firing temperature for each specimen is shown in Table 3.

The ceramic disks prepared by firing at the optimum firing temperature were provided on its opposite sides with silver electrodes by applying silver paste and then baking it at 800° C. to prepare specimens for measurements of electrical properties.

For each composition, measurements were made on relative dielectric constant (ε) at 20° C., 1 KHz and 1 Vrms, dielectric loss factor (tan δ) at 20° C., 1 KHz and 1 Vrms, distortion factor, and temperature characteristics of capacitance (TCR of C) over the temperature range of -25° C. to +85° C. relative to the capacitance at 20° C. Results are shown in Table 3.

In Table 3, the distortion factor is given by the third harmonic distortion measured by applying a radio frequency voltage (10 KHz) of 100 Vrms/mm or 200 Vrms/mm to the specimen.

In Tables 1 to 3, the specimen with an asterisk (*) are those out of the scope of the present invention, whereas other specimens are those falling within the scope of the present invention.

                                      TABLE 1                                      __________________________________________________________________________     Composition (wt %)                                                                                                  Vitreous                                  Main component (wt %)                component                                 composition (wt %)            Additive                                                                              Compo-                                    No SrTiO.sub.3                                                                        PbTiO.sub.3                                                                         CaTiO.sub.3                                                                         Bi.sub.2 O.sub.3                                                                   TiO.sub.2                                                                         SnO.sub.2                                                                            Re.sub.2 O.sub.3                                                                      sition                                    __________________________________________________________________________      1 34.7                                                                               21.6 5.2  25.4                                                                               8.3                                                                               4.8                                                                               97.0                                                                              Re = Nd                                                                             1.0                                                                              A    2.0                                   2 37.2                                                                               15.2 12.3 19.6                                                                               6.4                                                                               9.3                                                                               96.0                                                                              Re = La                                                                             2.0                                                                              B    2.0                                   3 41.2                                                                               16.5 18.2 10.2                                                                               12.4                                                                              1.5                                                                               95.0                                                                              Re = Ce                                                                             3.0                                                                              C    2.0                                   4 38.1                                                                               14.9 14.0 21.1                                                                               7.4                                                                               4.5                                                                               97.0                                                                              Re = Pr                                                                             1.0                                                                              A    2.0                                   5 35.0                                                                               18.9 16.5 16.5                                                                               6.0                                                                               7.1                                                                               95.5                                                                              Re = Sm                                                                             2.5                                                                              B    2.0                                   6*                                                                               18.2                                                                               35.2 18.3 15.2                                                                               6.8                                                                               6.3                                                                               97.0                                                                              Re = Nd                                                                             1.0                                                                              A    2.0                                   7*                                                                               53.1                                                                               8.2  10.5 17.0                                                                               8.4                                                                               2.8                                                                               93.0                                                                              Re = La                                                                             3.0                                                                              A    4.0                                   8*                                                                               47.3                                                                               5.8  5.2  25.6                                                                               9.8                                                                               6.3                                                                               91.0                                                                              Re = Sm                                                                             2.0                                                                              A    7.0                                   9*                                                                               24.0                                                                               40.3 8.2  12.4                                                                               8.4                                                                               6.7                                                                               89.5                                                                              Re = Nd                                                                             0.5                                                                              A    10.0                                 10*                                                                               25.2                                                                               25.2 3.0  32.4                                                                               8.2                                                                               6.0                                                                               95.0                                                                              Re = Ce                                                                             1.0                                                                              A    4.0                                  11*                                                                               24.5                                                                               12.5 35.0 13.7                                                                               12.3                                                                              2.0                                                                               89.0                                                                              Re = Pr                                                                             4.0                                                                              A    7.0                                  12*                                                                               48.3                                                                               25.4 10.3 3.5 10.5                                                                              2.0                                                                               95.5                                                                              Re = Sm                                                                             2.5                                                                              B    2.0                                  13*                                                                               23.0                                                                               13.2 9.4  38.0                                                                               11.8                                                                              4.6                                                                               81.0                                                                              Re = Nd                                                                             4.0                                                                              B    15.0                                 14*                                                                               43.4                                                                               17.2 15.3 20.2                                                                               1.5                                                                               2.2                                                                               87.0                                                                              Re = Pr                                                                             3.0                                                                              C    10.0                                 15*                                                                               23.1                                                                               22.0 6.2  30.4                                                                               15.2                                                                              3.1                                                                               89.5                                                                              Re = La                                                                             0.5                                                                              C    10.0                                 16*                                                                               22.0                                                                               33.3 8.4  25.1                                                                               11.2                                                                              0  97.0                                                                              Re = Nd                                                                             1.0                                                                              C    2.0                                  17*                                                                               29.9                                                                               13.5 6.5  22.1                                                                               13.0                                                                              15.0                                                                              96.0                                                                              Re = Sm                                                                             2.0                                                                              C    2.0                                  18*                                                                               22.4                                                                               33.2 6.6  27.3                                                                               10.1                                                                              0.4                                                                               98.0                                                                              --   0 B    2.0                                  19*                                                                               43.2                                                                               9.3  16.6 17.2                                                                               7.3                                                                               6.4                                                                               90.0                                                                              Re = La                                                                             8.0                                                                              B    2.0                                  20*                                                                               34.7                                                                               21.6 5.2  25.4                                                                               8.3                                                                               4.8                                                                               99.0                                                                              Re = Ce                                                                             1.0                                                                              --   0                                    21*                                                                               37.2                                                                               15.2 12.3 19.6                                                                               6.4                                                                               9.3                                                                               80.0                                                                              Re = Nd                                                                             3.0                                                                              B    17.0                                 22*                                                                               41.2                                                                               16.5 18.2 10.2                                                                               12.4                                                                              1.5                                                                               93.0                                                                              Re = La                                                                             2.0                                                                              D    5.0                                  23*                                                                               41.2                                                                               16.5 18.2 10.2                                                                               12.4                                                                              1.5                                                                               90.0                                                                              Re = Sm                                                                             5.0                                                                              E    5.0                                  24*                                                                               41.2                                                                               16.5 18.2 10.2                                                                               12.4                                                                              1.5                                                                               94.0                                                                              Re = Pr                                                                             1.0                                                                              F    5.0                                  25*                                                                               41.2                                                                               16.5 18.2 10.2                                                                               12.4                                                                              1.5                                                                               94.5                                                                              Re = Ce                                                                             0.5                                                                              G    5.0                                  26*                                                                               41.2                                                                               16.5 18.2 10.2                                                                               12.4                                                                              1.5                                                                               94.5                                                                              Re = Nd                                                                             0.5                                                                              H    5.0                                  27*                                                                               41.2                                                                               16.5 18.2 10.2                                                                               12.4                                                                              1.5                                                                               93.0                                                                              Re = La                                                                             2.0                                                                              I    5.0                                  28*                                                                               41.2                                                                               16.5 18.2 10.2                                                                               12.4                                                                              1.5                                                                               94.5                                                                              Re = Sm                                                                             1.0                                                                              J    5.0                                  29*                                                                               35.2                                                                               24.4 5.7  28.2                                                                               4.2                                                                               2.3                                                                               92.0                                                                              Re = Ce                                                                             3.0                                                                              K    5.0                                  30*                                                                               35.2                                                                               24.4 5.7  28.2                                                                               4.2                                                                               2.3                                                                               94.0                                                                              Re = La                                                                             1.0                                                                              L    5.0                                  31*                                                                               35.2                                                                               24.4 5.7  28.2                                                                               4.2                                                                               2.3                                                                               92.5                                                                              Re = Ce                                                                             2.5                                                                              M    5.0                                  32*                                                                               35.2                                                                               24.4 5.7  28.2                                                                               4.2                                                                               2.3                                                                               93.5                                                                              Re = Nd                                                                             1.5                                                                              N    5.0                                  __________________________________________________________________________

                                      TABLE 2                                      __________________________________________________________________________     Composition of Vitreous component (mol %)                                      Ref.                                                                               Li.sub.2 O                                                                        BaO                                                                               MgO                                                                               CaO                                                                               SrO                                                                               SiO.sub.2                                                                         MnO.sub.2                                                                          TiO.sub.2                                                                         CuO                                                                               Al.sub.2 O.sub.3                               __________________________________________________________________________     A   27 6  0  5  5  45 3   4  3  2                                              B   27 6  0  0  5  47 2   3  2  8                                              C   27 4  3  4  4  44 3   4  3  4                                              D*   9 7  2  7  7  54 1   6  2  5                                              E*  46 4  1  4  4  30 2   3  3  3                                              F*  30 0  0  0  4  55 2   3  3  3                                              G*  15 10 10 10 15 34 1   2  1  2                                              H*  42 4  6  6  6  25 1   3  3  4                                              I*  15 2  0  0  4  65 3   4  3  4                                              J*  32 5  5  6  6  13 12  14 3  4                                              K*  39 7  2  7  7  34 0   0  1  3                                              L*  30 8  2  8  8  32 1   6  0  5                                              M*  28 6  3  6  6  42 2   4  3  0                                              N*  24 5  3  5  5  41 1   2  2  12                                             __________________________________________________________________________

                                      TABLE 3                                      __________________________________________________________________________        Firing   tan δ                                                                       Third harmonic                                                                         Distortion (dB)                                         No.                                                                               temp (°C.)                                                                    ε                                                                         (%)                                                                               100 Vrms/mm                                                                            200 Vrms/mm                                                                            TCR of C (%)                                    __________________________________________________________________________      1 1140  1410                                                                              0.35                                                                              -88     -76     -10                                              2 1160  1280                                                                              0.28                                                                              -94     -83     -8                                               3 1120  1240                                                                              0.25                                                                              -92     -81     -11                                              4 1160  1470                                                                              0.36                                                                              -84     -75     -11                                              5 1140  1360                                                                              0.32                                                                              -88     -79     -12                                              6*                                                                                960  1950                                                                              2.73                                                                              -55     -48     -20                                              7*                                                                               1140   320                                                                              0.48                                                                              -92     -78     -30                                              8*                                                                               1080   240                                                                              0.27                                                                              -96     -81     -22                                              9*                                                                                940  2320                                                                              3.20                                                                              -52     -46     -21                                             10*                                                                               1140  1610                                                                              1.77                                                                              -79     -70     -17                                             11*                                                                               1120   420                                                                              0.35                                                                              -85     -73     -25                                             12*                                                                               1180   450                                                                              0.80                                                                              -87     -76     -24                                             13*                                                                                980  1470                                                                              2.12                                                                              -79     -69     -12                                             14*                                                                               1140   440                                                                              1.46                                                                              -81     -70     -13                                             15*                                                                               1120  1000                                                                              1.61                                                                              -82     -74     -8                                              16*                                                                               1100  1820                                                                              1.85                                                                              -57     -50     -26                                             17*                                                                               1160  1010                                                                              2.21                                                                              -84     -74     -14                                             18*                                                                               1080  2020                                                                              1.86                                                                              -62     -53     -18                                             19*                                                                               1120   430                                                                              1.48                                                                              -86     -75     -23                                             20*                                                                               1240  1700                                                                              0.43                                                                              -84     -71     -9                                              21*                                                                                940   380                                                                              0.88                                                                              -77     -69     -14                                             22*                                                                               1220  1830                                                                              0.52                                                                              -83     -74     -12                                             23*                                                                               1040   570                                                                              1.73                                                                              -85     -75     -11                                             24*                                                                               1220  1660                                                                              0.61                                                                              -86     -76     -13                                             25*                                                                               1220  1580                                                                              0.52                                                                              -87     -76     -13                                             26*                                                                               1240  1720                                                                              0.48                                                                              -85     -76     -12                                             27*                                                                               1200  1380                                                                              0.62                                                                              -86     -75     -14                                             28*                                                                               1240  1420                                                                              0.55                                                                              -85     -76     -13                                             29*                                                                               1220  1030                                                                              0.38                                                                              -87     -77     -10                                             30*                                                                               1160  1100                                                                              1.71                                                                              -86     -76     -16                                             31*                                                                               1220  1080                                                                              0.67                                                                              -84     -68     -13                                             32*                                                                               1240  1050                                                                              0.42                                                                              -82     -74     -14                                             __________________________________________________________________________

As will be understood from the above results, the dielectric ceramic composition according to the present invention has a high dielectric constant of not less than 000, a dielectric loss factor of not more than 0.5% at 1 KHz, a low distortion factor and a small change rate of capacitance on temperature. In addition, it is possible to fire it at a relatively lower temperature of not more than 200° C.

The main composition has been limited to those having a compositional proportions defined as above for the following reasons: If the content of SrTiO₃ is less than 20.0 wt %, the dielectric loss factor becomes larger than 0.5%, and the distortion factor becomes large (specimen No. 6). If the content of SrTiO₃ exceeds 50.0 wt %, the relative dielectric constant becomes less than 500, and the temperature change rate of dielectric constant becomes large (specimen No. 7).

If the content of PbTiO₃ is less than 8.0 wt %, the relative dielectric constant becomes less than 500 (specimen No. 8). If the content of PbTiO₃ exceeds 37.6 wt %, the dielectric loss factor becomes larger than 0.5%, and the distortion factor becomes large (specimen No. 9).

If the content of CaTiO₃ is less than 3.2 wt %, the dielectric loss factor becomes larger than 0.5% (specimen No. 10). If the content of CaTiO₃ exceeds 33.9 wt %, the relative dielectric constant becomes less than 500 (specimen No. 11).

If the content of Bi₂ O₃ is less than 4.4 wt %, the relative dielectric constant becomes less than 500 (specimen No. 12). If the content of Bi₂ O₃ exceeds 35.2 wt %, the dielectric loss factor becomes larger than 0.5% (specimen No. 13).

If the content of TiO₂ is less than 2.5 wt %, the relative dielectric constant becomes less than 500 (specimen No. 14). If the content of TiO₂ exceeds 13.6 wt %, the dielectric loss factor becomes larger than 0.5% (specimen No. 15).

If the content of SnO₂ is less than 0.2 wt %, the dielectric loss factor becomes larger than 0.5%, and the distortion factor becomes large (specimen No. 16). If the content of SnO₂ exceeds 12.0 wt %, the dielectric loss factor becomes greater than 0.5% (specimen No. 17).

The dielectric ceramic composition of the present invention has been limited to those containing 0.1 to 5.0 wt % of at least one oxide of rare earth elements, and 0.2 to 15 wt % of the glass component incorporated in the main component for the following reasons:

If the added amount of rare element oxide (Re₂ O₃) is less than 0.1 wt %, the dielectric loss factor becomes larger than 0.5% (specimen No. 18). If the content of rare earth element oxide exceeds 5.0 wt %, the relative dielectric constant becomes less than 500 (specimen No. 19).

The added amount of glass component has been limited a value from 0.2 to 15.0 wt % for the following reasons. If the added amount of the glass component is less than 0.23 wt %, the sintering temperature becomes higher than 1300° C. (specimen No. 20). If the added amount of the glass component exceeds 15.0 wt %, the ceramics becomes porous and the dielectric constant becomes less than 500 (specimen No. 21).

The glass component incorporated into the main component has been limited to those having a composition defined as above for the following reasons.

If the content of Li₂ O in the glass component is less than 10 mol %, the sintering temperature becomes high (specimen No.22). If the content of Li₂ O exceeds 45 mol %, the melting point of the glass component becomes lower than 800° C., thus making impossible to use it as a sintering auxiliary agent. In addition, the dielectric loss factor becomes more than 0.5% (specimen No.23).

If the content of at least one oxide of BaO, MgO, CaO and SrO is less than 5 mol % or exceeds 40 mol %, the sintering temperature becomes higher than 1200° C. (specimens No. 24 or 25).

If the sum of the content of SiO₂ and that of at least one oxide of TiO₂ and MnO₂ is less than 30 mol % or exceeds 70 mol %, or if the sole content of SiO₂ is less than 15 mol %, or if none of TiO₂ and/or MnO₂ is incorporated in the glass component, the sintering temperature becomes higher than 1200° C. (specimens No. 26, 27, 28 and 28).

Copper oxide CuO incorporated into the glass component contributes to reduce the dielectric loss and temperature characteristics of the product. If the content of CuO is less than 1 mol %, the dielectric loss factor becomes more than 0.5% (specimen No. 30). If the content of CuO exceeds 35 mol %, the insulating resistance becomes considerably lowered.

If the content of alumina (Al₂ O₃) in the glass component is less than 0.2 mol %, or if its content exceeds 10 mol %, the sintering temperature becomes higher than 1200° C. Thus, the content of Al₂ O₃ in the glass component has been limited to from 0.2 mol % to 10 mol %. 

What is claimed is:
 1. A dielectric ceramic composition consisting essentially of 80 to 99.7 wt % of a main component of a system SrTiO₃ -PbTiO₃ -CaTiO₃ -Bi₂ O₃ -TiO₂ -SnO₂, 0.1 to 5.0 wt % of at least one oxide of rare earth elements in terms of Re₂ O₃, where Re is at least one rare earth element selected from the group consisting of Nd, La, Ce, Pr and Sm, and 0.2 to 15 wt % of a vitreous component,said main component consisting essentially, by weight, of 20.0 to 50.0% of SrTiO₃, 8.0 to 37.6% of Pb-TiO₃, 3.2 to 33.9% of CaTiO₃, 4.4 to 35.2% of Bi₂ O₃, 2.5 to 13.6% of TiO₂, and 0.2 to 12.0% of SnO₂, said vitreous component consisting essentially of 10 to 45 mol % of Li₂ O, 5 to 40 mol % of at least one oxide selected from the group consisting of BaO, MgO, CaO and SrO, 0.2 to 10 mol % of Al₂ O₃, 30 to 70 mol % of at least two oxides selected from the group consisting of SiO₂, MnO₂ and TiO₂, and 1 to 35 mol % of CuO, the content of SiO₂ in the vitreous component being not less than 15 mol % at the least. 